1. Field of the Invention
The present invention relates to a heavy duty tire in which bead durability is improved by restricting blowout of carcass cords while also restricting deformation of bead cores at bead portions of bead-wind structure.
2. Description of the Related Art
A carcass ply a that constitutes a framework of a tire is generally arranged, as illustrated in FIG. 9(A), in that it is engaged by providing turnup portions a2 which are turned over around the bead cores b, on both sides of a ply main body portion a1 that bridges between bead cores b, b. For the purpose of improvements such as weight saving of the bead cores, it has been recently suggested a structure as disclosed in Japanese Published Patent Application No. 11-321244 of a heavy duty tire employing a carcass ply a constituted from steel cords in which, as illustrated in FIG. 9(B), ply turnup portions a2 are continuously formed at the ply main body portion a1 in which the ply is wound around the bead cores b by approximately one round. Such a structure is referred to as a so-called “bead-wind structure”, and this term is used also in the present specification.
In such a structure, the ply turnup portions a2 end around the bead cores b. This will accordingly be of advantage in that hardly any stress acts onto tip ends a2e thereof at the time of deformation of the beads so that damages such as cord loosening originating at the tip ends a2e can be effectively prevented.
At the same time, the ply turnup portions a2 are wound at a small radius of curvature in such a structure. As a result, so-called spring back is apt to strongly occur in which the carcass cords (steel cords) try to restore their original different shape, and the shape of the carcass cords tends to be unstable. Particularly in case such spring back happens during green tire molding processes, clearances are apt to be formed between the ply turnup portions a2 and the bead cores b and defective moldings such as air residues are caused.
On the other hand, it would be possible to deform the ply turnup portions a2 through a modeling means in this “bead-wind structure” for firmly winding the ply around the bead cores b. In such an instance, the ply turnup portions a2 constraint the bead cores b as illustrated in FIG. 9(B). Through tension force F that is applied to the carcass cords, a strong rotation moment M is generated around the bead cores. This rotation moment M causes core deformations such as arrangement confusions (alignment shifts) of the bead wire around the bead cores b. It may also make a contact pressure between corner portions p on bead toe sides of the bead cores b with the carcass cords becomes large to cause fretting that may lead to breaks in cords.
Particularly in the bead-wind structure, the bead core b is deformed in a direction in which it rotates around the sectional center thereof through the rotational moment M. For restricting such rotational deformation, it will be desirable to achieve flattening of the bead cores. However, such flattening of the bead cores will cause a tendency in encouraging bead deformation and occurrence of fretting.
Accordingly, Japanese Published Patent Application No. 2001-246675 and Japanese Published Patent Application No. 2000-219016 have suggested forming a bent portion k are preliminarily formed at the carcass ply a through plastic deformation in which carcass cords are partially bent and molded at positions that suit corner portions of the bead cores b as illustrated in FIG. 9(C). However, when partial molding (partial deformation using a die) of the carcass cords is performed, the cord strength will be degraded through the partial bending to result in early rupture of the cords, and it will become difficult to secure sufficient bead durability.
In this respect, Japanese Published Patent Application No. 2002-59716 discloses an arrangement in which a reinforcing layer is provided outside of the ply turnup portions for preventing blowout by clamping the ply turnup portions.